Can identical DC power supplies with common ground produce varying voltage on same circuit?

For large installations of 5V LEDs, this PixLite page suggests using "power injection" from supplemental power supplies. Their wiring sample says:

Note that the DC ground of all power supplies used should be connected together to provide a single common ground reference. In addition to this, the positive output from each power supply should be isolated in order to avoid differing voltage potentials occurring across multiple power supplies, which could result in damage.

I am trying to imagine a way in which "damaging" voltage (which I assume would mean significant overvoltage) could be developed by injecting from identical power supplies without isolation on the positive side if they share the same ground reference. Is that possible, and if so then how?